#include <bits/stdc++.h>
using namespace std;
int k;
string a;
int main(){
	cin >> k >> a;
	int cnt = 1;
	char n=a[0];
	for(int i = 1;i <= a.size();i++){
		if(a[i] == a[i-1]){
			cnt++;
		}else{
			n = a[i];
			cnt=1;
		}
		if(cnt == k){
			cout << n << endl;
			return 0;
		}
	}
	cout << "No" << endl;
}
